A press machine (a so-called electric servo press machine (a press machine); hereinafter, the press machine is also referred to simply as a press) for transmitting rotation of an electric servomotor, which is electronically controlled, to a slide and converting the rotation into vertical reciprocating movement of the slide through an intermediation of a power transmission/conversion mechanism (for example, a crank mechanism) so as to use the vertical reciprocating movement of the slide to perform press-working on a workpiece is known.
For the electric servo press machine as described above, the consideration as follows is required in view of advantages thereof (a free motion is enabled by the servomotor, and a flywheel and a clutch/brake unit provided to a conventional mechanical press may be eliminated at the same time).
Specifically, the conventional mechanical press has a configuration in which a motor (or the flywheel) corresponding to a driving source and a crank shaft may be physically (mechanically) completely separated from each other by a state of switching of the clutch/brake unit.
On the other hand, in view of an advantage that an operating state may be relatively freely controlled by using software, a further reduction in device cost and in size and the like, the electric servo press generally adopts a configuration that does not allow the physical separation between the driving source and an operating part while the servomotor and the crank shaft are constantly placed in a connected state.
In the electric servomotor, it is generally extremely difficult to reliably maintain and ensure a stop state when the electric servomotor is stopped in a controlled state (is placed in a servo-lock state) or to ensure that the electric servomotor is reliably stopped within a predetermined time period in the case where the servomotor should be stopped. Specifically, it is difficult to perfectly prevent runaway of the servomotor or the like.
In particular, in the case where the electric servo press is used in a hand-in-die operation, that is, the electric servo press is stopped for each stroke so that the workpiece is manually introduced and removed for use, if the electric servomotor and hence the slide move when the electric servomotor and the slide should be stopped, there is a fear of bringing about a situation where human physical safety is directly threatened. Therefore, the construction of a more advanced system which may realize a reliable safe stop is demanded.
In Patent Document 1, the electric servo press including a mechanical brake for complementing a servo brake or a dynamic brake or as braking means in place of the servo brake or the dynamic brake is proposed.
According to the electric servo press described in Patent Document 1, the addition of the mechanical brake having a larger braking force than that of the servo brake or the dynamic brake enables a more rapid stop and the maintenance of the stop state so as to prevent unexpected start-up or the like and therefore provide safety.
In the press described in Patent Document 1, however, the mechanical brake is operated for each stop. As a result, friction discs are worn to cause a problem in that the friction discs are required to be regularly replaced.
Further, for preventing the unexpected start-up or the like, the braking force of the mechanical brake is required to be larger than a maximum torque of the servomotor. Thus, the brake is increased in size. Moreover, in consideration of the need of the regular replacement of the friction discs increased in size, there is a fear of an increase in economic, burden.
Moreover, in Patent Document 2, an electric servo press for interrupting power to the servomotor to prevent the unexpected start-up (rotational drive) or the like due to the runaway of the servomotor or the like when an operator intrudes into a predetermined range while the press (the rotation of the motor) is in a stop state is proposed.
The electric servo press described in Patent Document 2 is devised so as to prevent a dangerous state from being brought about due to an erroneous operation, the runaway of the servomotor, or the like by the interruption of the power to the servomotor when a hand of the operator or the like intrudes into a press-working area (specifically, a dangerous area) during a setup operation or the like.
Specifically, the stop state of the electric servo press described in Patent Document 2 is more reliably maintained during the stop state of the press (the rotation of the motor). However, the case where an abrupt stop request is made during the operation of the press so as to immediately stop the press is not taken into consideration. Therefore, if a structure described in Patent Document 2 is directly used for the abrupt stop during the operation of the press, there is a fear in that, for example, the operation due to an inertia force is continued for a while.
Therefore, when the human hand or the like intrudes into the dangerous area during a press operation, there is no guarantee that the slide of the press is reliably stopped before the human hand or the like reaches the dangerous area. Thus, there is a fear that a human is physically harmed in a significant fashion. In particular, in the press including the power transmission/conversion mechanism of the press, which consists of the crank mechanism or the like, there is fear that the press may continue operating for a while due to the inertia force of the slide or the crank even after the power to the servomotor is interrupted to cause the driving force to disappear. Therefore, there is a fear in that the risk of an accident causing injury or death is further increased.
In Patent Document 3, a press machine for determining the occurrence of an abnormality when a difference between a position of a slide detected by a motor shaft-side encoder and that detected by a crank shaft-side encoder is equal to or larger than a set value is proposed.
Further, in Patent Document 4, a runaway monitoring device for a press, which monitors the amount of difference between values detected by a slide-side linear scale, a main gear-side encoder, and a motor shaft-side encoder so as to determine the occurrence of an abnormality is proposed.
It is certain that the abnormality such as a failure of the slide-side, crank shaft-side, or motor shaft-side encoder or the like is one of the factors which lead to the runaway of the servomotor, and therefore, it is effective to detect and address the abnormality to prevent the runaway. However, the runaway of the servomotor occurs not only due to the abnormality described above and may also occur due to, for example, the abnormality of a motion controller computing section of the servomotor or a storage section of motion control or the like. Therefore, there is a fear that the runaway monitoring device described in Patent Document 4 is insufficient as a countermeasure against the case where the human is physically harmed.
In Patent Document 5, a runaway monitoring device for detecting a press speed each time a predetermined period of time elapses after a deceleration stop command signal is input to a servomotor and for actuating mechanical braking when the press speed exceeds a preset speed is proposed.
The runaway monitoring device described in Patent Document 5 monitors a deceleration condition of the servomotor, and may effectively monitor not only the abnormality of the encoder as in the case of Patent Documents 3 and 4 but also the runaway occurring due to the abnormality of the computing section of the motion control, the storage section of the motion control, or the like.
However, the runaway monitoring device described above may determine the occurrence of the abnormality only after detecting that the speed has not been reduced to a preset speed at a time, at which the speed should have been reduced to the predetermined speed if the servomotor operates normally. Only after the determination of the occurrence of the abnormality, the mechanical brake is operated. Thus, the actual braking is started by the mechanical brake to start decelerating the servomotor after a delay corresponding to the sum of a time period required for the detection and a brake actuation time period from the input of a braking start command to the start of the actual braking by the mechanical brake. As a result, a stop time is ultimately delayed by the amount of delay. Moreover, if the servomotor is in a runaway state where the servomotor is driven at an increased speed or the like, the time period required for the braking is further increased. Therefore, the stop of the servomotor, and consequently, the stop of the press machine are further delayed.    Patent Document 1: JP Laid-Open No. 2003-290997 A    Patent Document 2: JP Laid-Open No. 2005-125330 A    Patent Document 3: JP Laid-Open No. 2003-205397 A    Patent Document 4: JP Laid-Open No. 2005-219089 A    Patent Document 5: JP Laid Open No. 2005-199314 A